3 III Preface This dissertation arose during my work at the Institut für Werkstoffe der Elektrotechnik (RWTH Aachen) and at Forschungszentrum Jülich. First, I would like to express my appreciation to Prof. Dr.-Ing. Rainer Waser for his guidance through the years of my doctorate and for giving me the opportunity to carry out research on a very exciting field. I am also indepted to Prof. Dr.-Ing. Stefan Heinen who kindly agreed to be co-examiner in the jury. I would like to thank Silvia Karthaüser from Forschungszentrum Jülich for her support and encouragement. I want to thank all colleagues at the Institute für Werkstoffe der Elektrotechnik and at Forschungszentrum Jülich for providing a pleasant working environment. Finally, I want to thank my wife Lubna for her support through these years of study. Jakob Mustafa June 2006

7 1 1. Introduction The performance of information equipment, such as personal computers and workstations, is improving dramatically. This improvement was the main driving force behind new memory generations with very high capacities and very high speeds. Today we have memory capacities which were unimaginable some decades ago and the future may bring capacities which are unimaginable for us. Portable devices like PDAs, digital cameras, and smart cards require in the first place non-volatile memories with very high capacities. Every advantage in some memory type is accompanied with one or more disadvantages. DRAMs have high capacities and relatively high speeds, but they are volatile and need to be refreshed every few milliseconds which raises their power consumption. SRAMs are the fastest memories but they are volatile and have the largest memory cell which reduces their capacities. Flash memories can have very high capacities and they are non-volatile but they are relatively slow. None of the existing memory technologies satisfies all of the requirements simultaneously. It is therefore common practice to combine two or more types together to compensate weaknesses of any single type. This combination means that different chips have to be used in the system or different technologies have to be implemented on a single chip. This will add to the cost of the system or it may not be possible to combine several technologies together. Another important issue concerning memories is scalability which is determined by economic, technological, and physical limits. A universal memory would be the solution for all or most memory problems. This memory would have a very high capacity, very high speed, very low power consumption and it would be non-volatile and scales better than existing technologies. Any new memory type with these specifications would face very hard challenges because it has to be superior to all well established technologies. In the recent years, there have been great advances in the areas of new materials, nanotechnology, and molecular electronics. A universal memory can benefit and utilize these concepts. In the area of materials, resistive hysteretic switching (bi-stable or multilevel switching) has got a great attention. Resistive hysteretic materials change their resistance from one state to another according to an applied voltage and its polarity. A binary value or more are represented through the state of the material s resistance. This concept of data storing differs fundamentally from conventional concepts which are mainly charge based. It also scales better than existing technologies. There is a great variety of resistive hysteretic materials which are still in the research phase. It

8 2 Introduction is still unknown which material is going to be the choice for the future. But all indications show that resistive memories are going to dominate. A great advantage of resistive memories is that they can be scaled down into the nanometer range without scarifying their functionality. Nanotechnology is especially attractive for the constructions of very dense passive memory arrays with a very low cost. A passive memory array will always have higher densities and will be cheaper than an active memory array and it will be easier to manufacture. Nevertheless, passive arrays need more complex periphery circuits to control them in comparison to active arrays. Resistive memories should not necessarily be made completely from new materials and technologies. As an intermediate solution, hybrid systems form current CMOS technology and the new technologies could be used. The next evolution step would be the combination of resistive materials with nano and molecular electronics. Resistive memory elements can also be used to build logical functions. This is especially attractive for FPGA designs where lookup tables, which consist of static or non-volatile memory cells, can be replaced with resistive memory elements. This work investigates utilizing hysteretic resistive elements in active and passive memories. Chapter 2 gives an overview of conventional memory technologies and emerging memory technologies. Chapter 3 is devoted for the analyses and simulation of resistive memories. In this chapter active and passive resistive memory designs are discussed. Some novel concepts concerning the design and optimization of resistive memories are also presented. Chapter 4 discusses the use of resistive elements to build logical functions in FPGAs. This work was a part of the MOLMEM (Molecular Memory) which is a joint research project between Research Center Jülich, Research Center Karlsruhe, RWTH Aachen University, and Infineon. In this project, new materials and concepts for future memories were investigated.

9 3 2. Semiconductor Memories Semiconductor memories exploit different physical states of a material or electronic device to represent information. Two well defined discrete states are needed at least to realize a memory function. The two states are assigned to the binary values 1 and 0. Utilizing more states is also possible. In this case more than one bit of information can be stored in the same physical space. Semiconductor memories are composed of three main architectural blocks: a cell array, a peripheral circuit, and an I/O unit circuit (Figure 1) [1]. m 2 Rows Address (n+m bits) I/O Interface Row Add. (n bits) Row Decoder cell n 2 Col. D/D in out Control Signals d/d in out Col. Address (m bits) I/O Control, Sense Amps Column Decoder Figure 1: Memory architecture The cell array contains the memory cells where the physical states are stored. The arrangement of the cells in an array structure reduces the number of control and sensing circuits since they are shared. An array comprising a matrix of 2 n rows and 2 m columns can store binary information of 2 n+m+k-1 bits if each element can store k bits. Any cell can be accessed randomly by selecting its corresponding row and column. Memory arrays can be passive or active. Active memory arrays utilize an active element, typically a select transistor, to access the memory cell. In contrast, passive array memories lack an active element in the array. The peripheral circuit is required to read and write the memory cells. This circuit includes address decoders, drivers, and sense amplifiers. The later is used to decide the states stored in the cells. The peripheral circuitry plays the role of a bridge between the memory cells and the I/O unit.

10 4 Semiconductor Memories The I/O circuit is the interface of the memory to the external world. This block converts external input signals such as addresses, data inputs, clock, and control signals to the corresponding internal signals that control the peripheral circuit. It also converts internal signals to external signals as in the case of reading information from the memory. There are several criteria, whereby semiconductor memories can be classified. For example, they can be classified according to the mechanism used to store information; like charge or polarization or resistance, or according to the material system used, or architecture, and so on. A common classification for memories concentrates on the write operation; like how fast a memory can be written to, or how many times write can be performed as can be seen in Figure 2. Semiconductor Memories RAM ROM volatile nonvolatile reprogrammable ROM once programmable ROM DRAM FeRAM EPROM mask ROM SRAM MRAM EEPROM PROM CNT RAM Flash PCRAM Nano Chrystal Traditional Technology Emerging Technology Figure 2: Categories of Semiconductor Memories Following this classification, semiconductor memories can be categorized in random access memory (RAM) or read only memory (ROM). In RAMs, information is read from or written to any location of the memory with virtually no limit on the number of reads and writes, whereas it can be written only once or few times and read unlimitedly in a ROM. Additionally, writing and reading speeds are almost identical in RAMs but they are very different in ROMs, since writing takes much more time than reading. The unshaded parts in Figure 2 represent standard well established memory technologies, while the shaded parts represent new emerging technologies. As can be seen, most of the emerging technologies fall under the category of RAM memories. All standard memory technologies and most of the emerging memory technologies are from the active array type. Only few types of the emerging memories comprise passive arrays. Both approaches have advantages and drawbacks. Active memories tend to be faster and have

11 larger memory arrays but they face limits because of scaling problems and manufacturing complexity imposed on semiconductor devices. Alternatively, passive arrays can be scaled aggressively and can be manufactured by relatively simple means, which makes them more suitable for future nanotechnologies. Passive arrays suffer from high cross talking between array elements, which limits their array size severely and complicates reading and writing. In the following sections, conventional and emerging technologies are discussed in more detail. 5

12 6 Semiconductor Memories 2.1 Conventional Memory Technologies Traditional memories are memories that use well established technologies and have been around for many years. The main principle of their function did not change over the years. They include volatile memories, once-programmable ROM memories, and most types of reprogrammable ROM memories Volatile Memories According to Figure 2, RAM can be further classified as volatile and non-volatile. A volatile memory loses the stored information if the power supply is turned off, which is the case in dynamic random access memories (DRAMs) and static random access memories (SRAMs) DRAM The memory cell of a DRAM consists of a transistor connected in series to a storage capacitor as seen in Figure 3. The bitlines of vertically adjacent cells are connected together as well as the wordlines of horizontally adjacent cells. Information is stored as a charge on the cell capacitor. The presence of a charge represents the logic value 1 and its absence the logic value 0. DRAMs have a low access time and very high density, which makes them suitable as main memories for digital devices. BL 1 Bl m WL 1 C C C C C BL C BL WL n C C C C Figure 3: DRAM memory cell array Data is written to a cell by holding the bit line at V DD or GND; afterwards, the cell transistor is activated, which causes the cell capacitor to be charged to V DD or discharged to GND respectively as illustrated in Figure 4.

13 Conventional Memory Technologies 7 BL WL write 1 write 0 Figure 4: DRAM write operation Reading is carried out by charging the parasitic capacitance of the bitline (C BL ) to V DD /2 then disconnecting it from the bitline driver which makes it float (Figure 5). V DD BL +v s V DD /2 -v s GND WL SA read 1 read 0 Figure 5: DRAM read operation After that the cell transistor is activated. As a result, the charge on the cell capacitor and the parasitic capacitance of the bitline will be shared. Depending on the state of the cell capacitor, the bit line voltage would be raised or lowered by v s. The voltage difference on the bitline before and after charge sharing (±v s ) is expressed as: v s VDD = 2 C c Cc + C BL (1) The voltage at the bitline is compared to a reference voltage using a sense amplifier, which is activated by the signal SA. The sense amplifier decides whether a 0 or a 1 is stored in the cell. According to the decision, the sense amplifier sets the level of the bitline to V DD or to GND to perform information write back. Since the cell capacitor is not ideal, leakage current in the capacitor will lead to discharging the cell capacitor within few milliseconds and the information will be lost. To solve this

14 8 Semiconductor Memories problem, all cell capacitors have to be refreshed before the charge falls to a level, which is not more detectable. This means, that after storing the information and while the charge is still detectable, the information is read and then written back to the memory cells SRAM An SRAM cell is essentially a flip-flop circuit as can be seen in Figure 6. The nodes N1, N2 have either the voltages V DD and GND, or GND and V DD respectively. In the first case, the cell contains a 1 and in the second case a 0. To store 1 in the cell, BL is hold at V DD and BLB at GND and the transistors T5 and T6 are activated by the wordline (WL). This will disturb the cell for a short time, after which the new state will be programmed. Storing a 0 is similar, but his time the voltages on BL and BLB are exchanged. WL BL BLB T5 N1 N2 T6 Figure 6: SRAM memory cell The read operation is performed by detecting the polarity of a differential signal voltage developed on the data lines BL and BLB. No refresh operation is needed because the leakage currents at N1 and N2, if any, are compensated by a static current from the power supply, as long as V DD is supplied. Compared to DRAMs, SRAMs have a much larger cell area but their access time is much shorter. For this reason SRAMs are used where access time is critical like in caches or supercomputer memories Programmable Memories ROMs can be classified as once-programmable or reprogrammable devices. As the name indicates, a once-programmable memory is programmed once during fabrication (mask-based ROM) or after fabrication (programmable read only memory: PROM). Reprogrammable ROM devices, on the other hand, can be erased after having been programmed by exposing them to ultraviolet light in case of erasable programmable read only memory (EPROM) or by electrical means in case of electrically erasable read only memory (EEPROM). All cells in an

15 Conventional Memory Technologies 9 EPROM are erased simultaneously, whereas they are erased byte-wise in EEPROMs or block-wise in Flash memories Mask ROM/PROM Memories Programming during fabrication is carried out by patterning the last metallization layer to define which memory cells are connected or disconnected to represent a 1 or a 0 as seen in Figure 7. BL WL cut Figure 7: Mask ROM cell In a PROM, the memory cells are always connected to GND through tiny fuses after fabrication. The connections can be broken by blowing the fuses. As it was the case by ROMs, the existence or non existence of a connection defines whether a 0 or a 1 is stored. Blowing a fuse is irreversible, which makes rewriting impossible EEPROM/Flash Memories EEPROMs and Flash memories are very similar. The main difference between them is that EEPROMs can be erased and programmed byte-wise but Flash memories are programmed and erased block-wise. EEPROMs require a memory cell consisting of two transistors whereas only one cell transistor is required in flash memories. In general, flash memories have replaced EEPROMs because their cell size is smaller and they can have very large capacities. The memory cell of a flash memory comprises a single MOS transistor with an additional floating gate as seen in Figure 8. The threshold voltage of the transistor is shifted depending on the trapped charge in the floating gate. The floating gate is electrically isolated from its environment. But still it can be charged and discharged. The two typical mechanisms to transfer electric charges from and into the floating gate are the hot electron injection and the Fowler-Nordheim tunneling. Electrical charge transfer is carried out during writing only. The threshold shift due to the electric charge is expressed as:

16 10 Semiconductor Memories ΔQ FG Δ VT = C (2) FC poly ONO SiO 2 poly Control Gate Floating Gate BL WL Figure 8: Flash memory cell where ΔQ FG is the shift of the floating gate charge and C FC is the capacitance between the floating gate and the control gate. To read the cell, a voltage V r, which is higher than low threshold and lower than the high threshold, is applied at the gate of the transistor as shown in Figure 9. If the cell has a low threshold voltage, current will flow in the transistor which will indicate that the stored value is 1 ; otherwise, if the cell s threshold is higher than the applied voltage, no current will flow which means that the cell contains a 0. V r V r S D S D low V T high V T Source-Drain Current 0 ON V T OFF V r low V T high V T Control Gate Voltage Figure 9: Flash memory read operation

17 Conventional Memory Technologies Nitride Storage Memories Scaling the tunnel oxide thickness of flash memory devices causes reduced reliability and data retention. The replacement of the floating gate with a localized trapping material can help alleviate this problem because only localized discharging takes place in the presence of oxide defects. SONOS (oxide-silicon nitride-tunnel oxide-silicon) flash memory cells have been proposed for years. The carriers are stored in the traps of the nitride layer between top and bottom oxide as shown in Figure 10 [2]. D/S Poly Gate Oxide Nitride Oxide D/S Figure 10: SONOS flash memory cell SONOS cells offer several advantages over conventional floating gate memory cells like ease of manufacturing, insensitivity to oxide defects, and the non existence of floating gate coupling effect. However, the cell retention, the cell size and slow program/erase speed are still an issue. Recently SONOS cells have evolved into a 2-bits storage architecture (Multi-Bit Cell) by utilizing the localized charge trapping effect of nitride, which enables a memory cell to hold twice as much data as a standard memory cell, without compromising device endurance, performance or reliability. Figure 11 shows where the two bits are stored in such a cell. Trapped charge memories can be used in either NOR or NAND memory architectures. Bit 1 Bit 2 WL BL Oxide BL Diffusion BL i-1 p-sub BL i Figure 11: Storing 2 bits in one memory cell Among numerously proposed cell architectures, NROM [3] and PHINES [4] are the most promising multi-bit cell Technologies. The structure of the two types is similar as can be seen in Figure 12. The main difference between NROM and PHINES is the way they are programmed and erased [5]. The NROM is programmed using channel hot electron injection (CHE) and is

19 Conventional Memory Technologies Multi Level Flash Memories Usually, one bit of information is stored on the floating gate of a flash memory. According to the stored charge on the floating gate, the threshold voltage of the cell transistor is shifted as can be seen in Figure 13a. In the case of the multi level cell, two bits of information can be stored on the floating gate of the cell transistor. To achieve this, four states of charges are needed as seen in Figure 13b. Number of Cells Number of Cells Cell Threshold Voltage [V] Cell Threshold Voltage [V] (a) (b) Figure 13: Single bit/cell array threshold voltage histogram [6] Precision is the key to store two bits per cell. Programming a cell (charge placement) and reading (sensing) must be precisely controlled in order to have four states within a single transistor. Higher bit per cell densities are possible by even more precise charge placement control. Three bits per cell would require eight distinct charge states.

20 14 Semiconductor Memories 2.2 Emerging Memory Technologies Almost all the emerging memory types are non-volatile memories. There is a great number of new memory concepts and materials that have been developed to replace conventional memory technologies. These emerging technologies focus on developing a universal memory which replaces all previous memory types. Future memories are non-volatile and reliable, additionally they have very high capacities, low power consumption, and very high data transfer rates FeRAM Ferroelectric materials are characterized by reversible spontaneous polarization of a certain class of materials. There are a wide variety of crystals that show this kind of behavior like PbZr x Ti 1-x O 3 (PZT), SrBi 2 Ta 2 O 9 (SBT) and (Bi,La) 4 Ti 3 O 12 (BLT) [7, 8]. Spontaneous polarization arises due to noncentrosymmetric arrangement of ions in their unit cells which produces a permanent electric dipole moment associated with the unit cell. Adjacent dipoles also tend to orient themselves in the same direction to form a region called ferroelectric domain. The polarization response with the electric field of these materials is highly non-linear and exhibits a hysteresis loop as shown in Figure 14, which shows a perovskite unit cell of a ferroelectric material and a typical hysteresis curve. P P r -E c E E c -P r Figure 14: Ferrolectric unit cell and hysteresis loop As applied electric field is increased, the ferroelectric domains which are favorably oriented with respect to the electric field grow at the expense of other domains. This continues until total domain growth and reorientation of all the domains is reached in a direction favorable to the external field. At this stage the material has a saturated polarization. If the electric field is removed at this point some of the domains do not reorient into a random configuration, thus leaving the material still polarized. This polarization is known as remnant polarization P r. The

21 Emerging Memory Technologies and Devices 15 strength of the electric field required to return the polarization to zero is called the coercive field E c. The net direction of the polarization is in the direction of the external electric field. If a capacitor configuration is used, two states of polarization can be achieved according to the applied field on the capacitor plates. From a digital point of view these two states can correspond to a stored 0 or 1. Ferroelectric materials can be used with a variety of memory architectures which are closely related to conventional memories like DRAM and Flash memories. A 1T-1C ferroelectric memory cell is shown in Figure 15. BL WL C BL CFE PL Figure 15: 1T-1C ferroelectric memory cell This cell is similar to a DRAM cell with the exception of the plateline (PL) which has a variable voltage level to enable the switching of the polarization of the ferroelectric capacitor (FeCAP), whereas its level is fixed in a DRAM. To write a 1 in the cell, the BL is set to V DD and the PL is grounded, then a pulse is applied at the wordline (WL) to activate the cell transistor (Figure 16a). Writing a 0 is accomplished in the same manner but this time the polarities of BL and PL are exchanged to reverse the polarization of the FeCAP (Figure 16b). BL=1 WL= BL=0 WL= C BL CFE C BL CFE PL=0 PL=1 a) Write 1 b) Write 0 Figure 16: FeRAM write operation Another possible architecture is the chain FeRAM (CFeRAM) [9] where the cell transistor and capacitor are connected in parallel and the cells are connected in series. Figure 17 shows two cell blocks of this memory type.

22 16 Semiconductor Memories BL BS 1 WL 11 WL 10 WL 20 WL 21 BS 2 PL Figure 17: Chain FeRAM This architecture is similar to that of a NAND-Flash and can achieve higher densities than the 1T-1C architecture but has a longer access time. In contrast to the conventional 1T-1C cell, accessing a CFeRAM cell is accomplished by grounding the cell s WL and applying V DD to all other neighboring cells, which short-circuits their corresponding FeCAPs. The voltage difference between BL and PL will only be dropped on the selected cell. To read the content of the memory cell first the BL is grounded, then it is made floating, which means that it effectively represents a capacitance C BL. After that the cell is selected by WL. Then PL voltage is raised from GND to V DD. This would raise the voltage of BL in dependence of the polarization (data) stored in the FeCAP and the capacitance C BL. Other cell architectures like the 2T-2C [9, 10] cell and the 1T cell (FeFET) are possible. In a 2T-2C cell one bit is stored in two capacitors that always have opposite polarizations. Despite the fact that this cell type is very reliable, it is not attractive because of the cell size which is twice the size of a 1T-1C cell. A 1T cell (FeFET) is in principle a MOSFET transistor whose gate dielectric is ferroelectric. An advantage of this cell type is that the reading operation is nondestructive but the main disadvantage is the fact that the current achievable retention time is very short to be used in a nonvolatile memory [11]. A ferroelectric memory diode where the diode resistance is modulated through a ferroelectric effect has also been reported [12, 13] MRAM The development of magnetoresistive random access memory (MRAM) has been based on a number of significant ideas over the past 20 years, starting with Cross-tie Random Access Memory (CRAM) [14, 15] and continuing with new configurations. All types of MRAM use magnetization direction for information storage and the resultant resistance difference for information readout. The change of resistance of a material due to a magnetic field is called megnetoresistance. The magnetoresistnace ratio is the quotient of the difference between maximum and minimum resistance values and the minimum resistance value. There are three types of magnetoresistance. The first type is the anisotropic magnetoresistance

23 Emerging Memory Technologies and Devices 17 (AMR), which is the change in resistance with the angle between the magnetization and the electric current in a ferromagnetic metal. Figure 18 illustrates the method of data storage in the MRAM cell using AMR. The cell consists of two ferromagnetic films sandwiching a poor conductor, with the composite film etched into stripes as shown. A current through the stripe magnetizes magnetic material clockwise or counterclockwise when aided by a current (field) from an orthogonal strip line. Current in either strip by itself would not change the storage state. Thus, a single memory cell could be selectively written in a 2D array. 0 State 1 State Magnetization Magnetization Word Line Word Line Figure 18: MRAM with AMR cell Reading of this cell depends on the differential resistance of the cell, when a sense current is passed through it. Because the sense current creates a magnetic field which opposes the magnetization in one storage state, but has the same direction in the other state, the angle of rotation is different for a 1 or 0. The magnetoresistance ratio of this cell type is about 2%. Despite improvements in reading methods [16], the maximum differential resistance of this cell when it is read is about 0.5%. In real arrays with practical sense currents, this gives differential sense signals of 0.5 to 1.0 mv. These sense signals allowed 16K bit integrated MRAM chips to operate with a read access time of about 250 ns [17]. The write time for the MRAM was 100 ns. The second type of magnetoresistance is the giant magnetoresistance. In this type data is stored in magnetic multilayer and the giant magnetoresistance (GMR) effect is used to read the stored data. This type has three times improvement of magnetoresistance than AMR and nine times improvement in reading access time. There are two types of devices that use the giant magnetoresistnce effect, which are the spin valve (Figure 19) and the pseudo-spin valve devices (Figure 20). Free Layer Spacer Pinned Layer Pinning Layer Figure 19: spin valve

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